A metal coping is used in dentistry in the construction of a dental crown and/or bridge. The metal coping functions as the under structure of the crown and is usually covered, for reasons of aesthetics, with a fired-on coating of a ceramic porcelain composition or a polymer based veneering material. The metal coping supports the coating and provides the required structural strength and rigidity for the restored tooth to resist the forces of mastication.
A metal coping may be cast from an investment of a wax or plastic pattern of the tooth to be restored. An alternative procedure which does not require waxing, investing or casting and which currently has been gaining wide acceptance by many laboratory practitioners and dentists is to form the coping from a moldable dental material composition composed of a mixture of high and low fusing temperature metal particles, as disclosed, for example, in U.S. Pat. No. 5,234,343 and U.S. Pat. No. 5,332,622 respectively. The dental material as taught in these patents, the disclosure of which is herein incorporated by reference, forms a porous structure upon heat treatment having a high void volume of above at least 20%. Before heat treatment the dental material is molded into the shape of the tooth to be restored. The molded shape is self-supported and is converted upon heat treatment in a dental furnace into a porous structure essentially any without shrinkage. The heat treatment temperature must be sufficient to entirely or substantially melt the low fusing temperature metal particles but rot the high fusing temperature metal particles. This results in the low fusing temperature metal particles interconnecting the high fusing temperature metal particles to form the porous structure without affecting the shape of the structure. The porous structure has a high void volume which is then filled to solidify the structure by adding a filler material of a metal or ceramic in a secondary heat treatment procedure.
In U.S. Pat. No. 5,593,305, the disclosure of which is herein incorporated by reference, it is further taught that the moldable dental material may be formed into a compacted strip formed of a base material of high and low fusing temperature metal particles and wax. Likewise the filler material may also be shaped into a compacted strip and composed of a composition of filler material and wax. The strip of base material is hand molded over the surface of a die and heat treated followed by a similar procedure for the strip of filler material in a secondary heat treatment operation.
It has been discovered in accordance with the present invention that a coping formed of two laminated metallic layers of high and low fusing temperature metal particle compositions can be heat treated in a dental furnace in a single operation with the heat treatment carried out gradually or in stages either manually or automatically. Heretofore, it was necessary to heat treat the base metallic layer of high fusing temperature metal particles separately to form an independent porous structure before adding filler material. The filler material was applied to the porous structure followed by a separate heat treatment operation to cause the porous structure to densify.
The method for forming a dental coping in accordance with the present invention comprises the steps of: forming a first sheet composed of a base material composition containing high fusing temperature metal particles and a wax binder; forming a second sheet of a filler material composition containing low fusing temperature metal particles and a wax binder, superimposing the two sheets to form a laminate, molding the sheets either separately or as a laminate over the surface of a die into the shape of a tooth to be restored, placing the molded laminate into a furnace and raising the temperature in the furnace from a temperature substantially below the melting temperature of the low temperature fusing particles to a maximum temperature not to exceed about 1075xc2x0 C. The furnace temperature should preferably be raised gradually or in stages and over a time period sufficient to cause the high fusing temperature metal particles to form a stable porous structure of high fusing temperature metal before a temperature is reached in the furnace sufficient to melt and densify the low fusing particles in the porous structure.